This application claims the priority of Korean Patent Application No. 2003-48428, filed on Jul. 15, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a screen for a projection display and, more particularly, to a projection display with a high definition view, slim structure and a large screen in which the light is uniformly transmitted throughout the screen even when an incident angle of light emitted from an optical projection system onto the screen is different depending on the position on the screen to which the light is transmitted.
2. Description of the Related Art
Referring to FIG. 1, a projection screen usually includes a Fresnel lens sheet 10 which collects light flux projected by an optical projection system toward a viewer, and a lenticular lens sheet 20 which disperses the light output from the Fresnel lens sheet 10 at a predetermined angle with respect to a horizontal direction of the screen, to widen a viewing angle.
A lenticular lens 20a is formed on an incident surface of the lenticular lens sheet 20 to disperse light in the horizontal direction. A black stripe 20b is formed on an exit surface of the lenticular lens sheet 20 to absorb the diverging light and to block the externally incident light, thereby increasing a contrast ratio.
Recently, projection displays having a large screen, a high definition view, and a slim structure have been researched and developed. In FIG. 2A, an optical projection system 25 is disposed at the same level as a center of the Fresnel lens sheet 10, with a plane incident surface 10a and an exit surface 10b. With this arrangement, it is necessary to decrease the height of a quadrangular pyramid of a light flux output from the optical projection system 25 to make the projection displays slim. However, when the optical projection system 25 is disposed at the same level as the center of the screen, the light flux output from the optical projection system 25 cannot be decreased to an optimal height of the quadrangular pyramid.
To solve this problem, the optical projection system 25 is disposed obliquely below the screen, as shown in FIG. 2B. However, in this case, the light loss increases due to a reflection at a peripheral portion of the Fresnel lens sheet 10, and the brightness at the peripheral portion of the screen decreases, thereby deteriorating a picture quality.
FIG. 3 is a graph illustrating the relationship between the transmittance of the Fresnel lens sheet 10 and an incident angle of light for the Fresnel lens sheet 10. Referring to FIG. 3, as the incident angle increases, the transmittance of the Fresnel lens sheet 10 rapidly decreases. In the graph illustrated in FIG. 3, T1 denotes an amount of light passing through a plane incident surface 10a of the Fresnel lens sheet 10 shown in FIG. 2, T2 denotes an amount of light passing through an exit surface 10b, i.e., a Fresnel lens surface, and TT denotes a product of T1 and T2.
As the light approaches the top of the screen, the incident angle of light increases more significantly in a system shown in FIG. 2B (referred to as a second system) than in a system shown in FIG. 2A (referred to as a first system). Accordingly, referring to FIG. 3, the light loss due to the reflection of light in the peripheral portion of the screen is greater in the second system than in the first system.
Consequently, when the light from an optical projection system 25 is obliquely incident onto a screen, as shown in FIG. 2B, a projection display can be made slim, but the picture quality deteriorates since a dark area and a bright area occur on the screen.